1. Field of the Invention
The present invention relates to a micro-oscillation element such as a micromirror, an acceleration sensor, an angular speed sensor and a vibration element. The present invention also relates to an array utilizing such micro-oscillation elements.
2. Description of the Related Art
Recently, elements having a minute structure formed by micromachining technology have been utilized in various technical fields. Examples of such elements include micro-oscillation devices such as a micromirror element, an acceleration sensor and an angular speed sensor, each of which comprises a very small oscillating portion. Micromirror elements are employed for reflecting light in the technical fields of optical disks or optical telecommunications, for example. The acceleration sensor and the angular speed sensor are employed, for example, for stabilizing an image in a digital video camera or a still camera incorporated in a mobile phone against the user's hand motion. These sensors are also employed for a car navigation system, an airbag inflation timing system, or for a posture controlling system of a car or a robot. Conventional micro-oscillation elements are described in JP-A-2003-19700, JP-A-2004-341364 and JP-A-2006-72252, for example.
FIGS. 21 to 23 depict a micro-oscillation element X3, as an example of the conventional micro-oscillation elements. FIG. 21 is a plan view of the micro-oscillation element X3, and FIGS. 22 and 23 are cross-sectional views taken along a line XXII-XXII and a line XXIII-XXIII in FIG. 21, respectively.
The micro-oscillation element X3 includes an oscillating portion 40, a frame 51, a pair of torsion bars 52, and a combtooth electrode 53, and is built up as a micromirror element. For the sake of explicitness of the drawings, the oscillating portion 40 and the frame 51 are hatched in FIG. 21.
The oscillating portion 40 includes a land portion 41, a combtooth electrode 42, and a beam portion 43. The land portion 41 includes on its surface a mirror portion 41a that reflects light. The combtooth electrode 42 serves as a movable electrode in the driving mechanism of the micro-oscillation element X3, and is constituted of a silicon material made conductive. The beam portion 43 connects the land portion 41 and the combtooth electrode 42. The beam portion 43 is constituted of a silicon material made conductive.
The frame 51 has such a shape that surrounds the oscillating portion 40, and is constituted of a silicon material. The frame 51 includes therein a predetermined conductive path (not shown).
The pair of torsion bars 52 defines an axis A3 of the oscillating motion of the oscillating portion 40, or the land portion 41. Each of the torsion bars 52 is connected to the beam portion 43 and the frame 51 of the oscillating portion 40, thereby serving as a link therebetween and, as shown in FIG. 23, is thinner than the beam portion 43 and the frame 51 in a thicknesswise direction H thereof. The torsion bars 52 also serve to electrically connect the conductive path in the frame 51 and the beam portion 43, and are constituted of a silicon material made conductive.
The combtooth electrode 53 serves to generate electrostatic force in cooperation with the combtooth electrode 42, and is fixed to the frame 51 as shown in FIG. 23. In other words, the combtooth electrode 53 constitutes a fixed electrode of the driving mechanism of the micro-oscillation element X3. The combtooth electrode 53 is constituted of a silicon material made conductive. The combtooth electrodes 42, 53 are located at different levels in height from each other as shown in FIGS. 22 and 23, for example when the oscillating portion 40 is not working. The combtooth electrode 42, 53 are also located such that the respective electrode teeth are disaligned from each other, in order to avoid interference when the oscillating portion 40 is driven to oscillate.
In the micro-oscillation element X3, giving a predetermined potential to the combtooth electrodes 42, 53 can rotationally displace the oscillating portion 40, or the land portion 41, about the axis A3. The potential can be given to the combtooth electrode 42 via the predetermined conductive path in the frame 51, the pair of torsion bars 52, and the beam portion 43, and the combtooth electrode 42 is grounded, for example. Upon generating desired static attraction between the combtooth electrodes 42, 53 by giving the predetermined potential to each of the combtooth electrodes 42, 53, the combtooth electrode 42 is attracted toward the combtooth electrode 53. Accordingly, the oscillating portion 40, or the land portion 41, oscillates about the axis A3, and can be rotationally displaced by such an angle that the static attraction between the electrodes and the total torsional resistance of the respective torsion bars 52 are balanced. The amount of the rotational displacement in such oscillating motion is controlled through adjusting the potential to be given to the combtooth electrodes 42, 53. Upon canceling the static attraction between the combtooth electrodes 42, 53, the respective torsion bars 52 restore the natural state, so that the oscillating portion 40, or the land portion 41, assumes the orientation as shown in FIG. 23. Driving thus the oscillating portion 40 or the land portion 41 to oscillate allows changing as desired the direction of light reflected by the mirror portion 41a provided on the land portion 41.
As noted above, the micro-oscillation element X3 drives the oscillating portion 40 to oscillate about the axis A3 defined by the torsion bars 52. Such configuration, however, makes the weight balance of the oscillating portion 40 about the axis A3 rather undesirable. Specifically, as shown in FIG. 21, the upper half of the oscillating portion 40 above the axis A3 in the drawing has a relatively dense structure, while the lower half below the axis A3 has a relatively sparse structure (in other words, the upper and lower halves of the oscillating portion 40 are nonsymmetrical). Further, as shown in FIG. 23, the axis A3 is biased to a lower position in the drawing in a thicknesswise direction H of the oscillating portion 40. Such structure impedes achieving desirable weight balance of the oscillating portion 40 about the axis A3. The poor weight balance of the oscillating portion 40 impedes accurate adjustment of the rotational displacement in the oscillating motion of the oscillating portion 40. For instance, the oscillating portion 40 is prone to slight rotation under the influence of the gravity. In an acceleration sensor or an angular speed sensor, the undesirable weight balance in the oscillating portion is degrading to the sensing characteristics.